1. Field of the Invention
The present invention relates, in general, to solid oxide fuel cells and, more particularly, to a single cell for solid oxide fuel cell stacks, being shaped while being bent downwardly at opposite two or four sides of the cell to form an electrode support type structure or a self-support (electrolyte support) type structure each having a reversed U-shaped cross-section, the present invention also relating to a solid oxide fuel cell stack structure with such single cells being gastightly stacked on a separating plate at a plurality of sealing grooves of the plate sealed with sealant.
2. Description of the Prior Art
As well known to those skilled in the art, fuel cells are designed to accomplish a smooth flow of reaction gases to two electrodes (i.e., anode and cathode), to bring the two electrodes into electric contact with an electrolyte substrate, and to accomplish a gastight sealing effect between the reaction gases. The fuel cells thus induce an ionic conduction from the electrodes toward the dense electrolyte substrate and create an electrochemical reaction in the electrodes, thereby forming electromotive force and finally generating electric power using the electromotive force.
In recent years, solid oxide fuel cells (hereinbelow, referred to simply as “SOFC”) have been proposed and used while being so-called “a third generation fuel cell”. In such an SOFC, a thermochemically stable metal oxide is used as the material of an electrolyte substrate, with a fuel electrode (anode) and an air electrode (cathode) being respectively attached to both (lower and upper) sides of the electrolyte substrate. Such an SOFC somewhat freely uses a variety of fuel gases, such as H2, CH4, CH3OH, etc., without reforming the fuel gases, and uses air or oxygen as an oxidant, thus effectively accomplishing a highly efficient and low pollution power plant.
A conventional SOFC stack consists of a fuel electrode (anode) (Ni—YSZ cermet), an electrolyte [doped zirconia (ZrO2+8Y2O3), doped ceria (CeO2), doped bismuth oxide (Bi2O3), doped perovskite], an air electrode (LaSrMnO3), a separating plate or an interconnector (Cr-5Fe-1Y2O3, Ni-based metal, LaSrCrO3), a current collector, and a sealant (glass or glass-ceramic). The above-mentioned elements are assembled into a desired SOFC stack. The SOFC stack is also assembled with peripheral equipment, thus accomplishing a desired power generating system.
Such an SOFC stack includes a plurality of single cells, each consisting of an electrolyte substrate with a fuel electrode as a negative electrode (anode) and an air electrode as a positive electrode (cathode) being attached to both sides of the electrolyte. In order to effectively create a desired electrochemical reaction in the two electrodes, the electrodes each preferably have a porous structure. In addition, the electrolyte substrate, or the intermediate layer of the single cell, preferably has a dense structure which does not allow fuel gas or oxidizing gas to permeate into the electrolyte or to be mixed together.
When such single cells are stacked into a desired SOFC stack, the single cells are positioned between two separating plates. In such a case, it is necessary to form a desired gastight sealing structure using a sealant, such as glass or glass-ceramic, within the stack so as to prevent two different gases from being mixed together while flowing along opposite gas channels of the separating plates. It is also necessary to design the SOFC stack to allow a smooth gas supply for the opposite electrodes of each single cell. In addition, an insulating layer or an insulating plate, made of a sealing and insulating material, is provided on an area of the upper separating plate, with the area being free from the single cells.
Conventionally, the SOFCs are classified into three types, such as a tubular type, a planar type and a monolithic type. Of the three types, the tubular type SOFC is the well-known type SOFC. However, such a tubular type SOFC is problematic in that it is very difficult to produce and is less likely to be practically used.
A known method of producing such a tubular type SOFC may be referred to a Minh's report (N. Q. Minh, J. Am. Ceram. Soc., 76[3] p 563-588, 1993). As disclosed in the above Minh's report, a porous electrode support in the form of a tube having a length of 2 mm is primarily produced through an extrusion process. Thereafter, a porous electrode layer is formed on the porous tubular support through a slurry coating process. In addition, both a desired electrolyte layer and a desired interconnector are formed through an EVD process (electrochemical vapor deposition process), thus producing a desired tubular type SOFC. The tubular type SOFC is somewhat advantageous in that it is easy and simple to accomplish both a desired gas sealing effect and an interconnection of single cells while stacking the tubular type SOFCs into an SOFC stack. However, the tubular type SOFC is problematic in that it has a low power density in comparison with the planar type SOFC or the monolithic type SOFC. In addition, it is necessary to enlarge the size and volume of EVD equipment in proportion to the length of a desired tubular type SOFC. This finally forces the EVD equipment to be large-sized and increases the equipment cost. Furthermore, a multi-step process has to be used for producing such a tubular type SOFC, thus increasing the production cost of single cells. Therefore, the tubular type SOFC will be less likely to be practically used.
Different from the tubular type SOFC and the monolithic type SOFC, the planar type SOFC is advantageous in that the electrolyte thin substrate having a thickness of 200 μm may be made of inexpensive conventional ceramic, thus being suitable for production in commercial quantity. Such a planar type SOFC also effectively improves the power density to an extent which cannot be expected from the tubular type SOFC or the monolithic type SOFC due to their structural disadvantages. In this regard, such planar type SOFCs rather than the tubular type SOFCs or the monolithic type SOFCs have been actively studied and developed recently.
Such planar type SOFCs are conventionally classified into electrode support type SOFCs and self-support (or electrolyte support) type SOFCs in accordance with the electrolyte, the electrode or the material. Of the two types, the self-support type (or electrolyte support type) SOFCs are more widely used rather than the electrode support type SOFCs. As shown in FIG. 1a, such a self-support type SOFC is produced by coating a positive electrode (cathode) layer and a negative electrode (anode) layer, each having a thickness of several ten micrometers, on both sides of an electrolyte substrate having a thickness of 200 μm. A known method of producing such a electrode support type SOFC may be referred to a Souza's report (S. de. Souza, J. Electrochem. Soc., 144[3] L35-L37, 1997). As disclosed in the above Souza's report, an electrolyte thin layer having a thickness of 20 μm is formed on a porous electrode support having a thickness of 1˜2 mm, thus forming a desired electrode support type SOFC having a highly improved electric performance. When an SOFC stack is produced using such electrode support type single cells, it is possible to preferably reduce the operational temperature of the SOFC stack from 1,000° C. to about 800° C. Therefore, the planar type SOFCs have been actively studied and developed recently to provide improved electrode support type SOFCs.
In order to accomplish the recent trend of high power capacity of SOFC stacks, it is necessary to assemble an SOFC stack having an enlarged area and this forces the area of each single cell of the stack to be enlarged. However, the known technique of producing ceramic thin plates only provides a square electrolyte plate having a dimension of about 10×10 cm or about 20×20 cm, the electrolyte plate being used as an electrode support plate. Therefore, as disclosed in a Blum's report (L. Blum et al, Proceedings of the 4th Int. Symp. On SOFC, Vol 4, p 163-172, 1995), a grid array SOFC stack, having a desired size larger than that of each single cell, is preferably proposed to be used as a planar type SOFC stack. In such a grid array SOFC stack, a plurality of single cells having a size smaller than that of a separating plate are arrayed in parallel on the separating plate while accomplishing a highly gas sealing effect as shown in FIG. 1b. However, such a highly gas sealing effect is very difficult to be accomplished during the process of producing the grid array stack, and so the requirement for the highly gas sealing effect stands in the way of practical use of such planar type SOFCs. The requirement for the highly gas sealing effect of the planar type SOFC stacks is very important since it directly determines the durability and expected life span of such stacks.